The present invention relates to a load-dependent asynchronous drive for a plurality of conveyors which are assembled to form a transport path, with the individual conveyors forming a common conveyor system. The conveyor system is provided with a suitable sensor system, such as light barriers, light scanners, inductive sensors etc. for controlling the individual conveyors.
Uncontrolled or free-running asynchronous motors typically have a problem in that the rotation speed of these drives is load-dependent. Accordingly, when asynchronous motors are used with conveyor systems, the transport speed disadvantageously depends on the weight of the transported goods. Moreover, the relative spacing between transported goods with different weight can change at the transition from one conveyor to the next. Since the relative displacement of the transported goods can in general not be predicted because their weight is typically unknown, these displacements create uncertainties which has to be taken into account when monitoring and processing the data. These uncertainties adversely affect certain variables, such as material flow, transport time to cover a planned transport path, and data security.
The following conventional methods are employed to eliminate the aforedescribed disadvantages:                a) Rotation speed feedback: in this case, the motor rotation speed or the speed of the conveyor belt are measured and deviations from the desired value are controlled. This increases the complexity of the measurement equipment and the controller.        b) Use of servo machines: in this case, drives are used where the characteristic curve is so steep that differences in the rotation speed are negligible. However, this solution is more expensive.        c) Use of synchronous machines: for controlling synchronous machines, the angular pole position, i.e., the rotor position, has to be known. The rotation speed of synchronous machines is advantageously synchronous to a preset value. However, the rotor position must disadvantageously be fed back, which is as complex as the feedback of the rotation speed discussed above with reference to a).        d) Use of electronically commutated DC machines: the same problems as described above with reference to c) apply here. i.e., the rotor position has to be known for commutation.        
It would therefore be desirable and advantageous to provide a simple low-cost and highly reliability drive control, which obviates prior art shortcomings and is able to specifically eliminate severe speed variations and the associated backup of the transported goods.